This invention relates to a system for regulating the NH.sub.3 -content of a physical solvent utilized in a gas absorption process for the removal of sour gases, especially carbon dioxide, from a crude gas containing minor amounts of ammonia, and in particular to preventing the precipitation of ammonium carbamate and/or a buildup of deleterious trace acids.
In the manufacture of industrial gases from crude gases, acidic components known as sour gases, especially CO.sub.2 and H.sub.2 S, must be scrubbed out of these crude gases. Suitable scrubbing media in this connection are, for example, physical scrubbing solvents including but not limited to alcohols, glycols, and ethers. The methanol scrubbing method is especially advantageous and is of commercial significance, the acidic components being scrubbed out with methanol at temperatures of down to below -70.degree. C. and under pressures of up to 100 bar. The methanol is not only used as a circulating physical scrubbing agent, but it is also used as an anti-freeze agent to prevent moisture present in the incoming crude gas from forming ice during the precooling of the gas. The resultant anti-freeze mixture of water and methanol is then separated from the gas and thereafter separated from each other in a methanol-water separating column.
The crude gas to be purified in most cases also contains traces of ammonia which become enriched in the methanol due to the very high solubility therein. Such an enrichment with ammonia may be desirable if the ammonia content can be kept constant, i.e. there is no excessive enrichment. However, frequently ammonia enrichment in methanol progresses to such an extent that, due to the simultaneous presence of carbon dioxide, solid deposits of ammonium carbamate are formed which in turn foul heat exchangers or clog conduits. The heat balance is also impaired by the heat of reaction of NH.sub.3 with the acidic gaseous components, and this disrupts the operation of the scrubbing process.
Patent application No. P 31 41 772.8 (Federal Republic of Germany,), now No. DE 3,141,772, owned by Linde, AG, the same assignee as the present invention, describes a process for the precipitation of ammonia from the scrubbing agent, e.g., methanol, with a precipitant, such as sulfuric acid. The process comprises several steps, such as metered feeding of acid, precipitation, cooling of the suspension, filtration or alternatively centrifuging, decanting or evaporation of the scrubbing medium, metered feeding of alkaline solution to buffer excess acid, transport of the filtrate to the methanol-water separating column, and a purifying cycle to remove the ammonium salt from the filter by water flushing. Owing to the number of process steps, and the apparatus involved, this process is more costly than desired.